1. Field of the Invention
This invention relates to an air separating system, and more particularly to an air separating system of the type which rectifies and separates oxygen and nitrogen by a double-column fractionating tower after cooling compressed feed air by reversing heat exchangers and which is provided with adsorbers for removing moisture, carbon dioxide and hydrocarbons such as carbon dioxide and acetylene and other impurities from the feed air by adsorption.
2. Description of the Prior Art
As an example, there has been known an air separating system such as illustrated in FIG. 1, where the feed air which is compressed to a predetermined pressure by a compressor 2 is precooled in an air cooling tower 3 and then fed to one of first and second adsorption columns 10a and 10b (e.g., to the adsorption column 10a) for the removal of moisture and impurities such as carbon dioxide. The purified air is cooled almost to its dew point by reversing heat exchangers 15 and fed to a lower column 16a of a double-column fractionating tower 16 for refinement and separation of oxygen and nitrogen, while drawing off excess nitrogen from a middle portion of upper column 16b. The excess nitrogen gas is partly released into atmosphere as waste nitrogen after heating to room temperature through heat exchanger 15, and further heated and partly fed to the other adsorption column (e.g., the adsorption column 10b) to serve as a regeneration gas. However, the air separating system of such arrangements is as a whole held in a highly pressurized stage due to the resistance of the adsorption column which uses the waste nitrogen gas for its regeneration, giving rise to the problem that the pressure of the feed air has to be raised to a level markedly higher than in a current air separation system without impurity adsorption columns. More specifically, the impurity adsorber of the type mentioned above requires a regenerelation gas pressure differential of about 0.3 Kg/cm.sup.2 across the adsorption column, which means an increase of about 0.7-0.8 Kg/cm.sup.2 in the pressure of the feed air which is generally about 5.0 Kg/cm.sup.2 G in an air separating system without impurity adsorption columns. Namely, the compressed feed air has to be supplied at a pressure of 6.0-6.4 Kg/cm.sup.2 G, which causes an increase of about 8-10% in the power necessary for the compression of the feed air. Such a pressure increase can be avoided by providing a blower for the regeneration gas. However, as a matter of fact, the blower-regeneration system is not generally accepted partly because of the complication of the system arrangements and partly because of the unreliability of the blower, although the power required for the operation of the blower is small enough.
In an air separation system with impurity adsorption columns as mentioned above, it is the general practice to precool the feed air to about 5.degree. C. to avoid the use of an adsorption unit of a large scale and to prevent increases in running costs. Nevertheless, the feed air is usually precooled by a refrigerator (ammonia or freon) which requires power of about 400 kw in a plant of a scale of 20000 Nm.sup.3 /h(O.sub.2).